(a) Field of the Invention
The present invention relates to an OFDM (orthogonal frequency division multiplexing) system. More specifically, the present invention relates to an AGC (automatic gain control) device and method in an OFDM system with a DC offset compensation function, and a recording medium storing a program containing the method.
(b) Description of the Related Art
AGC and the DC offset calculations are typically sequentially performed in a training sequence interval in the conventional OFDM system. Such AGC and DC offset calculations in the conventional OFDM system will now be described.
FIG. 1 shows a preamble configuration of the IEEE 802.11a WLAN, which is one type of an OFDM system.
As shown in FIG. 1, the preamble of the IEEE 802.11a includes a short training sequence and a long training sequence. The short training sequence is used for signal recognition, AGC, and coarse frequency offset estimation, and the long training sequence is used for fine sequence sync acquisition and fine frequency offset estimation.
These training sequences have normalized power, but data sequences do not have constant energies because the data sequences are obtained by performing IFFT (inverse fast Fourier transform) on data. Accordingly, it is necessary to perform AGC by using training sequences in the preamble interval.
Korean published application No. 2002-090562 discloses “An automatic gain control device of orthogonal frequency division multiplexing signals, and an automatic gain control method using the device”. This patent is characterized in that a two-stage AGC is executed using a digital AGC device. However, there is no disclosure in this patent of the affect of DC offsets in the AGC device.
An AGC device of the general OFDM system determines the energy of input I and Q data, calculates a mean value thereof, converts the mean value into a dB value that will be compensated for by the AGC device, and performs feedback of a difference between the converted dB value and a reference value to control the gain in the training sequence. The short training sequence is divided into a plurality of repeated intervals for the calculation of frequency offsets, and a mean value of each interval is 0. The long training sequence is divided into two repeated intervals (not including a CP, or cyclic prefix), and a mean value of each interval is 0. The mean value of the CP interval is not guaranteed to be 0, and in the case of the IEEE 802.11a, the mean value of the CP interval of the long training sequence is not zero and instead is a very large value. Since the data sequences (except the training sequences) are results obtained by performing IFFT on random values, the mean value for a predetermined interval is not constant and is a very large value. Therefore, the mean value is needed to calculate the DC offset and cancel the same in the training sequence interval.
Also, a device for determining the DC offset in the general OFDM system performs its operation by simply accumulating input I and Q data for a predetermined interval and taking a mean value of the accumulated data.
The general OFDM system performs AGC while assuming the DC offset to be an ignorable small value, after which the system calculates the DC offset to cancel the same. However, a drawback of this process is that it causes a reduction in accuracy in the initial sync acquisition stage. The DC offset problem is made worse if using the direct conversion method (i.e., converting RF, or radio frequency, signals directly into baseband frequency signals without using an IF, or intermediate frequency band), which is currently being developed for use in low price receiving systems.
Also, radio LANs do not load information on a subcarrier corresponding to a DC frequency to reduce or negate the affect of DC offset. Although such a method works well in the frequency domain, it reduces accuracy when performing operations in the time domain (e.g., initial synchronization and AGC).
Further, as shown in FIG. 2, which illustrates an actual time domain waveform of an OFDM system, since the intensity of the training sequence is less than that of the data sequence by roughly 10 dB, the training sequence is very sensitive to even low levels of DC offset. For example, when the maximum voltage intensity of input data is 2 Vp-p, the maximum voltage intensity of a training sequence is approximately 0.2 Vp-p. If a DC offset of 0.01V is generated in this case, although this corresponds to merely 0.5% of the input data, it corresponds to a much larger error of 5% with respect to the training sequence.